Interpretive Summary: One way to reduce the volume of organic wastes and to recycle nutrients is to mix the materials under controlled conditions and let them partially decompose. This process, called composting, can be used with many materials that have different physical properties. How the compost ingredients transfer moisture and especially heat are important factors in knowing how to manage the compost to obtain the right conditions for thorough decomposition and optimum nutrient recovery. Temperature is also important as it determines whether pathogens are killed during the composting process. This study involved laboratory measurements of the thermal properties of 12 compost materials. Three different measurement techniques were used to measure how much heat is held in the materials and how fast they conduct heat. Measurements were made with the compost having different moisture content and packed at different densities. The results showed a great deal of variation in thermal properties between materials but, when expressed based on the amount of pores filled with water, a more clear relationship became apparent. As water holds more heat and transfers heat more effectively than air, the thermal properties of compost increased with an increasing number of water-filled pores. This information is important to growers and policy makers as it demonstrates that management of the compost, especially the water content and density, has a significant effect on the temperature and effectiveness of the compost process.

Technical Abstract:
Thermal properties of compost bulking materials affect temperature and biodegradation during the composting process. Well-determined thermal properties of compost feedstocks will therefore contribute to practical thermodynamic approaches. Thermal conductivity, thermal diffusivity, and volumetric heat capacity of 12 compost bulking materials were determined in this study. Specific heat was measured by a differential scanning calorimeter. Transient heat dissipation and steady-state gradient methods were used for thermal conductivity and diffusivity measurements. Thermal properties were determined at varying bulk density, particle size, and water content. Thermal conductivity and volumetric heat capacity showed a linear relationship with moisture content and bulk density, thermal diffusivity showed a nonlinear relationship. Since the water, air, and solid materials have their own specific thermal property values, thermal properties of compost bulking materials vary with the rate of those three materials by changing water content, bulk density, and particle size. The degree of saturation was used to represent the interaction between volumes of water, air, and solids under the various combinations of moisture content, bulk density, and particle size. The first order regression models developed in this paper represent the relationship of degree of saturation versus thermal conductivity and volumetric heat capacity well. The resulting thermal properties of compost bulking materials can be used to develop heat transport models for the design of more optimal temperature control in composting systems.